1. Field of the Invention
The present invention relates to a process for producing a high purity caprolactam, and more particularly, to an efficient process for producing a high purity caprolactam, which comprises separating methylcyclopentanol contained in cyclohexanol prepared by the hydration of cyclohexene, which is obtained through partial hydrogenation of benzene, and removing efficiently methylcyclopentanone contained in cyclohexanone to be provided subsequently in oximation.
2. Description of the Prior Art
Cyclohexanone which is used in the manufacture of high purity caprolactam is prepared generally by dehydrogenation of cyclohexanol. The yield of this reaction depends on the various reaction conditions and a kind of a catalyst used, but in general is 40-90% of conversion at a temperature of from 100 to 400.degree. C., and an atmospheric pressure of 0-10. At this reaction, the catalyst may be used in powder form or in particulate form, but a particulate form catalyst is more desirable so as to obtain a better result. In general, these catalysts are used alone without using a carrier, or in conjunction with a known carrier, if necessary.
On the other hand, the general process for producing cyclohexanol includes a process comprising oxidation of cyclohexane prepared by hydrogenation of benzene to provide a mixture of cyclohexanol and cyclohexanone and a process comprising converting benzene into cyclohexene through partial hydrogenation, and subsequent hydration of cyclohexene.
In the former process, the oxidation product of cyclohexane that is prepared in the form of a mixture of cyclohexanol and cyclohexanone is produced by oxidation of cyclohexane using a gas including a molecular oxygen as an oxidant in a liquid phase. In this response, conversion and selectivity may be controlled by using a supported or non-supported catalyst system. However, there is economic disadvantage in recovering cyclohexanol because of a low conversion of the reaction. In addition, the ratio of the resultant alcohol to ketone must be controlled carefully upon preparing a mixture of cyclohexanol and cyclohexanone obtained through oxidation of cyclohexane.
In common, the ratio of alcohol exceeds the ratio of ketone.
In the latter process, partial hydrogenation of benzene by using a transition metal catalyst and co-catalyst system in an aqueous solution phase produces cyclohexene which is then hydrated by the inorganic solid acid catalyst to provide cyclohexanol. Advantageously, the partial hydrogenation reaction is carried out in such a way that benzene is mainly converted into its main reaction product, cyclohexene, while production of cyclohexane, which is a byproduct of the reaction, is suppressed by contacting the benzene with hydrogen gas in the presence of any catalyst selected form the catalysts described hereinafter.
EP 552, 809 A1 discloses a particulate hydrogenation catalyst comprised mainly of metallic ruthenium, and more particularly, a mixture of a ruthenium catalyst using a zinc compound as its co-catalyst and an oxide or a hydroxide of a metal such as silica, alumina, zirconium, or hafnium or the like which is used as a dispersing agent for increasing selectivity and accomplishing stability of the catalyst. On the other hand, examples of a catalyst for hydration of cyclohexanol include an inorganic acid(British Patent Nos. 1,381,149 and 1,542,996), hetero polyacid(Japanese Patent Publication SHO 58-1089), organic acid(Japanese Patent Publication SHO 43-16125) or zeolite(Japanese Patent Publication SHO 194828), or the like. Out of the above-mentioned catalysts, zeolite is desirable because it can provide advantages such as separation of catalyst and product and suppression of a side reaction.
A process for producing cyclohexanone through a dehydrogenation of cyclohexanol is more advantageous as compared to a process for producing cyclohexanone through a dehydrogenation of a mixture of cyclohexanol and cyclohexanone prepared by an oxidation of cyclohexane in that it can provide savings in production cost and more stability in production process. Accordingly, much attention is paid to the former process.
In spite of the above-mentioned advantages, the process for the production of cyclohexanol by the partial hydrogenation of benzene and subsequent hydration of the cyclohexene has a drawback that leads to a formation of undesired impurities such as methylcyclopentanol, cyclohexyl-cyclohexene isomer, and dicyclohexyl ether in cyclohexanol. These impurities are produced in an amount of 0-1000 ppm according to process conditions and are known to be produced by an isomer reaction or a dimerization or an etherification reaction between the partial hydrogenation and hydration. Cyclohexyl-cyclohexene isomer and dicyclohexyl, out of the aforementioned impurities, are high-boiling point compounds, so they can be easily removed during the cyclohexanol dehydrogenation process or by a column for removing a high-boiling point compound and a low-boiling point compound which is provided in front of a distillation column for separating a mixture of cyclohexanol and cyclohexanone. In contrast, there is significant difficulty in removing the methylcyclopentanol because its boiling point is almost the same as that of the content of a reactor. In the case that the methylcyclopentanol is converted to methylcyclopentanone and is fed to an oximation process, a purity of caprolactams produced through a Beckmann rearrangement and the quality of an ammonium sulfate by-product will tend to be deteriorated.
As a method for overcoming these problems, International Patent Publication 97/WO03956(Japanese Patent No. 9031052) describes a process for the production of .epsilon.-caprolactam capable of reducing the methylcyclopentanone content of the cyclohexanone to be converted into the oxime to 400 ppm or less by providing an additional distillation column to a conventional distillation process or by adopting a strict distillation condition so as not to subject methylcyclopentanone to be fed subsequently in oximation. However, this process suffers from considerable technical shortcomings, since providing the additional distillation column and adopting the strict distillation condition entail high maintenance costs and operation costs. Moreover, the quality of a caprolactam produced by this process cannot meet the quality requirement since the methylcyclopentanone is not removed completely.
Thus, the present inventors have repeated studies in order to overcome the above problems encountered in the prior art, keeping in mind that the methylcyclopentanol contained in the cyclohexanol was converted to the methylcyclopentanone, and if the methylcyclopentanone was fed to oximation, it became difficult to remove and affected adversely the subsequent process and quality of the product, whereas methylcyclopentanol itself did not affect the subsequent process and quality of the product unlike methylcyclopentanone. Consequently, we discovered that removal of the methylcyclopentanol from the cyclohexanol prior to use of such cyclohexanol in dehydrogenation so as not to form methylcyclopentanone through dehydrogenation improves the overall efficiency of the process and quality of the caprolactam.